The invention relates to friction welding methods and articles made by such methods. More particularly, the invention relates to methods for spin welding a plastic piece having a susceptibility to centrifugal imbalance, to the spin welding devices for practicing such methods, and to certain novel articles which can be produced by such methods, viz. seamless overpacks.
An important application of the present invention includes a novel spin welding apparatus and spin welding method for making novel plastic overpacks for use in the ultimate disposal, transportation and storage of industrial wastes that may cause harm to man and the environment. One important such application of the invention involves overpacks designed specifically to encapsulate 55-gallon steel drums by the spin welding apparatus and method of this invention, thus providing the means to stabilize corroding drums that are or would be leaking toxic wastes. Such drums populate public dumps and uncontrolled disposal sites in the United States as well as the premises of industrial operations and present a serious health hazard. Additional applications of the inventions address other objectives in waste management, such as the transportation, temporary storage, and ultimate disposal of unconfined toxic substances and contaminated soils. Such materials can arise from waste treatment processes and accidental release of hazardous materials. Some pollutants should be stabilized by solification in situ prior to sealing the overpack. In such cases, the contaminants would be secured by encapsulation employing the plastic overpacks and the friction welding apparatus of this invention.
Spin welding is a method for welding together two heat fusible plastic pieces. During the spin welding process, one heat fusible plastic piece is rotated at a specified speed while contacting the second piece at a specified pressure. Frictional heat is generated by the combination of contact pressure and spinning motion. As the spinning process continues, frictional heat raises the temperature of the heat fusible plastic until melting occurs. The continued application of presure causes continued melting and mixing of the heat fusible plastic. The continued application of pressure and the continued molting process will result in the formation of flash thrown from the rotating piece after the contact space between the two pieces becomes filled with the melt material.
After the contact space becomes filled with melt material, the spinning process can be stopped. After the spinning process stops, the melted heat fusible plastic is allowed to cool and to cure. Contact pressure between the two pieces is continued during this curing process. During the curing process, the melted heat fusible plastic solidifies and forms a weld between the first piece and the second piece. The weld is seamless in that the melt material from both pieces was mixed during the spinning process so as to form a gradient between the two pieces. Also, the weld is seamless because there is no air in the contact space containing the melt material. The melted heat fusible plastic within the contact space cures as one seamless unit, i.e. the melted plastic cools and solidifies as one unit seamlessly spanning across the two pieces and filling the contact area between the two pieces so as to join the two pieces without discontinuity.
An early review of basic spin welding technology was provided by Alex S. Neumann and Frank J. Bockhoff in their publication entitled, Welding of Plastics, pages 99-109 (Chapter 6--Friction Welding), Reinhold Co., N.Y., 1959. A somewhat later article, written by Robert M. Stemmler, Plastics Technology, pages 42-43, 1963, provides further information on the practice of spin welding methods. Both Neumann and Stemmler provide operable ranges for rotational speeds and contact pressures for obtaining a successful spin weld using small pieces having various thermoplastic compositions. Both Neumann and Stemmler taught that the spin welding process can generate sufficient heat to melt a particular heat fusible plastic for a successful weld only within these operable ranges. These operable ranges provided a lower limit for the rotational speed and contact pressure which can be used in a spin welding process.
A common type of spin welding device uses a drive shaft to which the rotated work piece is concentrically attached. Examples of such spin welding devices are given by Birkhold (U.S. Pat. No. 3,993,519) and Flax (U.S. Pat. No. 3,446,688). Even at the lowest range of rotational speed and pressure taught by Neumann and Stemmler, large work pieces can quickly become centrifugally unstable when concentrically attached to drive shaft. The prior art for such devices merely accepted these centrifugal imbalances and thereby imposed a practical limit on the size of the work piece. Spin welding large work pieces with such devices was precluded by the operating ranges taught by Neumann and Stemmler and by the susceptibility to centrifugal imbalance inherent to such large work pieces within these ranges. The present invention enables the use of large work pieces in a spin welding process using a drive shaft for concentrically driving the work piece.
Centrifugal imbalance can arise in a work piece from at least three possible sources: (a.) asymmetrical distribution of mass about an axis of rotation, (b.) mismatch between the axis of symmetry of the work piece and the axis of the drive shaft, and (c.) asymmetrical contact between the first piece and the second piece. In most cases, for a given geometry, the centrifugal imbalance of a work piece increases approximately with the square of the radius of the work piece.
To a degree, the susceptibility to centrifugal imbalance can be minimized for a given application by finely balancing and accurately centering the work pieces and by carefully leveling or matching the welding surfaces of each work piece. Minimizing the susceptibility to centrifugal imbalance enlarges the size limit for work pieces which can be spin welded. To a degree, such precautions are in fact taught and practiced by the prior art. However, the difficulty of implementing such precautions increases with the size of the work piece so that there are practical and commercial limits to the precautions which can be taken for increasing the size of the work piece. The practical size limit for the radius of a work piece used in prior art concentrically driven spin welding processes is estimated to be approximately four inches. Work pieces used in peripherally driven and supported spin welding methods may exceed this limit (discussed infra). The present invention teaches how to exceed this size limit using a concentrically driven spin welding processes.
Paolini (U.K. Pat. No. 1,460,752) describes one approach for spin welding large diameter pieces (pipes). Paolini uses an intermediate member to join two larger members. The intermediate member is rotated and supported within a cradle and driven at its perimeter. In the Paolini device, there is no drive shaft concentrically connected to the work piece to suffer the large moments generated by an imbalanced work piece. In the Paolini device, the vibrational energy generated by driving a centrifugally imbalanced intermediate member is dissipated by the cradle at the perimeter of the device. The configuration of the Paolini device directs the burden of supporting a centrifugally imbalanced work piece to support members at the perimeter of the work piece. Such forces are more easily handled at the perimeter than at the center. However, the Paolini method teaches only spin welding where the axis of rotation for the work piece lies in the horizontal plane. The present invention teaches that it is preferred that the work piece be rotated about a vertical axis during the welding process. Indeed, a chief product of the present invention requires that the work piece be rotated around a vertical axis.
The spin welding apparatus of the present invention uses pillow blocks to control the contact between the welding surfaces of the work pieces. The present invention uses a drive shaft to which one work piece is concentrically mounted and driven. The pillow blocks serve to damp vibrations occurring in the drive shaft caused by the centrifugal imbalance of a work piece. However, the chief purpos of the pillow blocks in the present invention differs from applications for pillow blocks found in the prior art.
In the prior art, pillow blocks were commonly used to damp vibrations so as to protect precision bearing surfaces supporting the drive shaft and to protect train elements connecting with the drive shaft. In the present invention, the pillow blocks are put to a novel use. In the present invention the pillow blocks serve to maintain the rotational concentricity of the rotating work piece so as to maximize its effective contact with the welding surface of the stationary work piece and so as to maximize the generation of frictional heat between the two welding surfaces. In the prior art, pillow blocks were used to minimize wear and frictional heat; in the present invention, the pillow blocks are used to maximize the generation of frictional heat and to accelerate the onset of melting.
The spin welding device of this invention enables the fabrication of seamless overpacks having a heat fusible plastic composition. A seamless overpack is a hermetically sealed container which lacks any seams. A seam is a joint between two abutting materials in which there is line discontinuity or other discontinuity which separates the two materials. A joint between two heat fusible plastic pieces which is properly formed by a spin welding process is said to be seamless. In a spin welding process, the spinning motion mixes the melted heat fusible plastic material within the contact space between the two pieces. Upon curing, there is a continous distribution gradient across the weld such that there is no line of discontinuity which separates the two materials. Although spin welding produces a macroscope weld line, close examination and testing reveals that the macroscope weld line is in fact seamless. Seamless welds are much stronger than welds having a seam.
The method for making the seamless overpacks of this invention includes and is dependent upon the spin welding method of this invention. Seamless overpacks having a heat fusible plastic composition are not found in the prior art. However, non-hermetically sealed containers having a heat fusible plastic composition and having a seamless spin welded joint are found in the prior art. These various containers are not hermetically sealed for a variety of reasons. One such example is a tubular container, of the type for tooth paste, described by Flax (supra). A second such example is a container, of the type for containing yogurt, described in a patent held by the Koehring Company (U.K. Pat. No. 1,316,151). Although both of these examples describe a spin welded joint, neither example is hermetically sealed. Both containers have an opening and include some type of detachable lid or screw top for covering and sealing the opening. Indeed, this opening is an essential element of these containers for at least three reasons: (a.) Since the seamless weld of these containers is made prior to filling the container, the opening is necessary to fill the container. (b.) If the methods described for spin welding these containers were used on a container containing its intented content, the content would spill from the container during the spin welding process. (c.) Facile means for opening and closing these containers is necessary to provide the consumer with convenient access to its content. Although these containers include a seamless weld, they are not hermetically sealed. The spin welded overpacks described in the present invention include hermetically sealed overpacks lacking an opening for accessing its content. The seamless overpacks of the present invention utilize the enhanced strength and intregrity of a seamless spin weld for hermetically sealing a large diameter opening for loading the overpack. The seamless overpacks of the present invention include overpacks which are sealed by the spin welding method while holding their enclosed content.
In the prior art, hermetically sealed containers have been made from materials other than heat fusible plastics. Hermetically sealed glass vials were known and used since at least the Renaissance period. Hermetically sealed lead containers were also known anciently. Both types of containers are hermetically sealed by fusing the glass or lead respectively with heat so as to form a seamless seal. Both types of hermetically sealed containers continue to find limited applications today. Due to the relatively high cost of these containers, the volume of the isolate is typically rather small and the value of the isolate is typically rather precious. The heat fusible plastic seamless overpacks of the present invention are used for inexpensively containing relatively large volumes of isolate. The initial application which stimulated the present invention involved the problem of isolating waste material contained by 55 gallon steel drums, where the continued integrity of the drum was suspect. The minimun radius for the aperture of such overpacks needs to be approximately 13 inches to accomodate these 55 gallon drums. Heat fusible plastic seamless overpacks are unknown in the prior art
There is no practical method taught by the prior art which could be applied to the problem of making wide mouth heat fusible plastic seamless overpacks having a strong seamless weld. Paolini (supra) teaches a spin welding method for joining two large diameter pieces of heat fusible plastic pipe with an intermediate member. However, it would be impractical to use Paolini's spin welding method for joining pipe so as to make overpacks. Paolini's method requires that the pipe lie horizontally. In the principal application for overpacks, it is undesirable to lie the 55 gallon drum on its side during the spin welding process. The typical isolate for an overpack is chemical waste material stored within a steel drum having suspect integrity. Lying the drum on its side within the overpack during the spin welding process could cause the drum to leak or spill. If the chemical waste is flammable, leaked waste material could catch fire due to the heat generated by the spin welding process. Paolini's spin welding would also be impractical for making overpacks because the method requires that two opposing horizontal pipe members abut the intermediate member with equal force. If two overpacks were substituted for the two opposing pipe members, the end product would be a double overpack joined by the intermediate member. Such a double overpack would be undesirable for a variety of reasons. It would be unwieldy; it would risk mixing the two enclosed waste materials; it would preclude the overpacking of a single drum and; it would preclude different storage handling of the two isolates. Although Paolini taught how to spin weld large pieces, he did not teach how to make a seamless overpack. Paolini's spin welding method is impractical for that purpose.